Our support for Bazaar, a third-party contribution, has depended on

their Python API. This change rewrites the module, modernizing it and

updating it to use the command line bzr tool instead. This keeps us

license-compliant and improves maintainability.
The one tricky part about Bazaar compatibility is that in order to use a

custom SSH program (rbssh in our case), we have to register it using the

Python API. To solve this, the rbssh support has been moved to a Bazaar

plugin, which is loaded by bzr on launch. Review Board sets environment

variables to tell bzr when it can find this. The plugin lives in the

Review Board source tree, but isn't part of the product, and instead

acts as its own top-level module living in the bzrlib.plugins

namespace (a requirement for Bazaar plugins).
Some other parts, like the revision handling, have been improved to be

more versatile, allowing for additional revision specifiers. The rbssh

support has also been simplified some, making use of the existing

RB_LOCAL_SITE environment variable instead of having to define custom

URLs with the Local Site name.
Additional unit tests have been written to more thoroughly test Bazaar

functionality.

Unit tests pass.
Tested unit tests without the bzr tool installed.
Tested functionality with a Bazaar repository.